I'm going through the same thing right now. We are going BM dress shopping next weekend and me and rest of the girls have yet to hear from one girl, including her own sisters. She is a FSIL though so I can't really kick her out. I'm just going on with my plans without her. If she shows up dressed at the wedding, great. if not, oh well.
I wouldn't be such a happy camper either, is there any way you could talk to her about it? Are you supposed to know that she's been blowing off your mom too? If so I would ask her what was going on, see if she's got an explanation for her crappy behavior. I agree, although you know how she is and it isn't an excuse, there's no reason to leave your mom hanging, that would be the point for me anyway, where I would get involved. Good Luck!!
I had a verrry simular situation with one of my BMs...finally she confessed that she couldnt' be in my BP...I was hurt... but time moved on. I would def. try meeting up with your BMs face to face to go over everything...even if it is only one girl at a time... JMO
